Centrifugal casting of tubes including slag separation

ABSTRACT

A method and apparatus for continuous centrifugal casting in which the mold is horizontally located and is rotating about its horizontal axis. One end of the mold is cooled to initiate solidification. Preferably, the interior of the tubing being cast is kept clean by ensuring that no slag enters the cooling zone and that the metal surface is shielded by a gas inert to the metal.

United States Patent [72] lnventors William Howard Considine 2,745,152 5/1956 Johnson l64/l l4 Storrington, Sussex; 2,770,857 1 H1956 Boissou 164/298 Derek Slater, l-loathly, Sussex, both of 2,940,l43 6/1960 Daubersy et al. l64/84 X England 3,367,400 2/1968 l-lathorn 164/84 [2]] Appl. No. 797,879 3,445,922 5/1969 Leghorn 164/84 X [22] Filed Feb. 10, 19,69 FOREIGN PATENTS [45] Patented Dec.7, 1971 960 26 31957 G I64 84 [73] Assignee The A.P.V. Company Limited x":

Sussex En land 22,708 III] 896 Great Br1ta1n l64/84 g 20,805 12/1961 Japan l64/66 Primary Examiner-J. Spencer Overholser CENTRIFUGAL CASTING 0F TUBES INCLUDING Assistant Examiner.lohn E. Roethel SEPQISATIiONF AlmrneyHolman & Stern alms, raw ng lgs.

[52] US. Cl l4 ABSTRACT: A method and apparatus for continuous Gem [51] In Cl Bzzd 13/10 trifugal casting in which the mold is horizontally located and is [50] Fieid 164/84 rotating about its horizontal axis. One end of the mold is 302 cooled to initiate solidification. Preferably, the interior of the v i tubing being cast is kept clean by ensuring that no slag enters 5 References Cited the coolilng zoneland that the metal surface is shielded by a gas UNITED STATES PATENTS e meta l,223,676 4/1917 Lavaud 164/84 x M41] I y 11k Z7 JJ Z alezslz'rs PATENTED 0E0 7197i SHEET 1 BF 2 IN VE N TORS mm N INVENTORS WILL/AM HOWARD CONS/D/NE DEREK SLATER a) W I f M ,4770P/v0 5 SHEET 2 [IF 2 D I I f \\\X\ H l H MU H H l H NM MMW/ W 7 m V \v V\ PATENTEU DEC 7 1971 CENTRIFUGAL CASTING OF TUBES INCLUDING SLAG SEPARATION BACKGROUND OF THE INVENTION This invention relates to centrifugal casting.

Centrifugal casting on a horizontal axis is currently used for the manufacture of tubes. However, the length of tube which can be manufacture is limited to the practicable length of a casting mould, e.g-. about feet, and lengths of tubing longer than this have to be made up by butt welding shorter lengths. This entails, cropping the ends of the shorter lengths back to a point of accurate internal and external diameter and then welding. There is thus a scrapping of material, which may be an expensive material or alloy steel such as stainless steel, and several time consuming and expensive process steps.

Further, the internal surface of the molten metal is covered with a slag or oxide layer during casting, and after solidification, this leaves a rough and often porous surface which must be cleaned by a machining operation.

Other methods of manufacturing tubes, such as solid drawing for thin walled tubing, or welding and drawing, also involve numerous process steps and so are expensive on plant and space as well as labor.

SUMMARY OF THE INVENTION The present invention consists in a method manufacturing tubing by centrifugal casting comprising continuously feeding molten metal into a mould rotating about a generally horizontal axis, cooling the metal adjacent one end of the mould whereby it solidifies into tubular form adjacent the one end, and continuously drawing off the solidified tubing from the one end of the mould.

The invention further consists in apparatus for continuously centrifugally casting tubing, comprising a mould mounted for rotation about a generally horizontal axis. Cooling means adjacent one end of the mould for solidifying molten metal into tubular form, and means for continuously drawing off the tubing so formed.

In order to produce tubing having a clean internal surface, a boring or machining tool may be inserted into the mould so as to clean the surface shortly after solidification.

An alternative approach to providing a clean internal surface is to ensure that the metal being solidified has a clean surface. This may be done by subjecting the metal to a centrifugal separation of metal and slag ahead of the cooling means in the mould and ensuring that only clean metal is fed to the zone of the mould adjacent the cooling means. For this purpose, the mould may be divided by a partition which has passages therethrough outside the interface between the metal and the slag layer. The slag may be allowed to flow over a weir at the end of the mould remote from the cooling means.

The partition may take the form of a spider or ported disc of refractory material attached to the wall of the mould, or it may be in the form of a rotary mandrel with a refractory disc coaxial with the mould.

Using a rotary mandrel, the disc would serve to seal the solidifying section of the mould, and the mandrel could be hollow' so as to allow the introduction of an inert gas into the solidifying section to protect the surface from oxidation by the atmosphere before solidification.

When compared with the conventional processes, the process according to the invention offers the advantages of eliminating cropping and welding, and when the slag is prevented from solidifying on the casting, the machining of the internal bore is also eliminated, together with the handling of the tube between all these stages.

Also, the continuous casting leads to modification of the solidification direction from radial to generally longitudinal, thereby improving the quality and density of the tube internal surface by the elimination of solidification shrinkage cavities.

For thin walled tubing, a drawing mandrel or rotary swagging stand may be located so as to operate on the tubing after it leaves the mould. The process would produce a seamless product metallurgically similar to solid drawn tube but avoiding the conventional series of steps, i.e., casting, boring or piercing, forging and drawing.

As compared with welded and drawn tube the six steps of casting, piercing, rolling, rolling to strip, forming, welding and drawing would be replaced by a one step operation and the product would be seamless as well.

BRIEF DESCRIPTION OF THE DRAWINGS DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS FIG. I shows a tundish 20 having a pouring nozzle 21 from which molten metal is fed to a feeder 22 to a mould 24 to be more particularly described hereinbelow. The mould is rotatable by means of drive rollers 25 driven by a motor 26. A mandrel rod 27 is shown supported at 28 and driven by a motor within the support 28. The tube 29 emerging from the mould 24 is shown as being cooled by spray nozzle 30 and then sub- 30 jected to planetary swagging my means of rolls 31, forming a planetary swagging stand, driven by a motor 32. The tube is withdrawn by inclined rollers 33 driven by a motor 34 and-is supported and moved on rollers such as 35.

FIG. 1a shows how the planetary swagging stand may be replaced by a rotating drawing die 36 driven by a motor 37.

As shown in FIG. 2, the mould 24 is in the form of a horizontal tube 1 spinning about its axis 2, which may be constructed either partially or wholly of copper, and water cooled by means of water or other fluid passing through a chamber 3. Molten metal 4 is continuously fed to a reservoir zone 5 at one end, and the produce tube is continuously extracted from a cooling or moulding zone 6 at the other, the extracted length of solidified product 7 spinning continuously and being parted off by a travelling parting tool or slitting wheel (not shown) into the lengths required. It will be evident that a continuous product of considerable length can be so manufactured.

The tube 7 is started on a dummy bar (not shown). This bar is made of sufficient mass and length to establish the truth of the relatively weak hot tube as it emerges from the mould. An accurately laid bed of driving rollers, of which only one roller 35 is illustrated in FIG. 1, extends for some distance from the mould zone, possible for 60 feet or so, right up to the point where parting takes place. The tube is constrained by a number of spring or piston controlled devices in order to en sure accurate contact with the driving rollers, thereby preventing bending or malformation of the tube as it emerges from the moulding zone in a still plastic state.

In continuous casting machines, it is necessary to maintain a small differential between the rate of travel of the mould and the rate of travel of the product. In order to ensure that the product does not adhere to the sides of the mould. This can easily be achieved by arranging a small differential between the rates of rotation of the mould and of the product.

As the product is rotated by means of the powered rollers 33 after being extracted from the mould, the rate of extraction is easily adjusted by introducing a slight angular offset to the rotating rollers 33.

In the most elementary system, the thickness of the tube is governed by the relation between the rate of extraction of the product, and the feed of metal to the mould, but problems of tube wall thickness control are evident.

The front end 8 of the mould 24 is constructed of refractory material, so that it contains a pool of liquid metal. The reservoir zone 5 thus created acts as a centrifugal separator, with cleaned liquid metal adjacent to the walls of the mould, and the slag 9 in the center. The mould 24 is arranged with a lip 10 of a suitable diameter so that the slag spills out over the lip, and is thereby eliminated from the process. The slag is prevented from passing forward into the solidifying section by means of a refractory plate 11, supported coaxially with the mould by a refractory spider 12 attached to the wall of the mould as illustrated. The combination of plate 11 and spider 12 may be replaced by a refractory disk and a rotating mandrel may be inserted through the center of the mould to support this disk. Clean metal then passes round the periphery of the disk 11, into the solidifying section 6 of the mould. Since the refractory disk forms a positive hermetic seal with the metal, the water-cooled section of the mould, in which solidification takes place, may be purged by an inert gas, so that a clean internal bore is obtained.

The thickness of the tube may then be controlled primarily by the depth of the liquid metal in the separating chambers, and may be adjusted by varying the position of the metal/slag interface. The position of this interface is a function of the position of the liquid surface in the solidifying chamber, and the position of this free, clean, metal surface may be accurately measured and controlled by means or a capacitance of other sensing device.

Additional adjustments to the position of this liquid surface may be carried out by varying the pressure of the gas in this chamber, provided the bore of the product was effectively closed. Alternatively, the thickness of the product may be controlled entirely by the lip of the separating chamber, by arranging that a small proportion of metal should spill with the slag, possibly through a notch, in order to direct the spilled metal to some point distinct from the bulk of the slag.

It should be pointed out that, if the internal bore of the lip is less than the required internal bore of the tube, then the radial head of metal in the solidifying section corresponds to the sum of the head of metal and slag in the separating chamber. If the diameter of the lip of the separating chamber is less than the required internal bore of the tube then when the system is in dynamic equilibrium the metal/slag interface will not rise to the lip of the separating chamber and no loss of metal will occur.

Dynamic equilibrium can be established by measuring the internal bore of the liquid metal pool in the solidifying chamber and controlling this by varying the admission of liquid metal to the separating chamber. This control would correspond to that by which the level of liquid metal in the mould of a continuous casting machine is controlled in conventional continuous casting practice.

This means that if a control loop of this type is established then no metal will be lost from the separating chamber, nor can the metal/slag interface fall to a point at which slag might enter the solidifying chamber.

Alternatively, it would be possible to control the internal surface of the tube by means of a water cooled mandrel, but this would give rise to considerable difficulties, as the product would tend to shrink on to this mandrel, and its position and temperature would therefore have to be accurately controlled.

It will be apparent that internal bore shrinkage is a negligible problem in continuous casting in view of the changed direction of the freezing front. However, the general characteristics of dendritic growth from the outer surface in a horizontal axis by continuousl introducin molten metal into the mould, continuously coo mg one en of the mould to solidify the metal at said one end into tubular form and continuously drawing off the solidified metal in tubular form from said one end; the improvement being the steps of centrifugally separating the molten metal from slag in a separating zone of the mould and subsequently passing the molten metal to the cooled end of the mould; preventing the slag from passing to the cooled end of the mould; and withdrawing the slag from the separating zone at an end thereof opposite the cooled end of the mould.

2. The method as claimed in claim 1 further comprising the step of introducing an inert gas into the mould adjacent the cooling means to protect the clean metal from atmospheric contamination.

3. The method as claimed in claim 1 further comprising the step of subjecting the metal in tubular fonn being drawn off from the mould to a drawing operation.

4. The method as claimed in claim 1 further comprising the step of subjecting the metal in tubular form being drawn off to a rotary swagging operation.

5. ln apparatus for the centrifugal casting of tubing including a mould rotatable about a horizontal axis, the mould having a feed end into which molten metal can be continuously fed, and a withdrawal end, the withdrawal end having means for cooling the withdrawal end and the metal passing therethrough whereby the metal is solidified into tubular form, and means for continuously withdrawing the metal solidified into tubular form; the improvement comprising separating means in said mould for centrifugally separating molten metal from slag, said separating means preventing passage of slag from the feed end into the withdrawal end; and peripheral weir means at the feed end'of the mould over which slag is withdrawn from the feed end as the level of slag increases.

6. Apparatus as claimed in claim 5 wherein said separating means comprises a partition dividing the mould and having through passages radially outwardly the interface between the metal surface and slag layer.

7. Apparatus as claimed in claim 6 wherein said partition is in the form of a refractory spider attached to the mould and a disc sealed to the spider.

8. The method as claimed in claim 1 wherein the thickness of the metal withdrawn in tubular form is controlled by varying the position of the metal/slag interface at the feed end.

9. The method as claimed in claim 8 comprising the further step of sensing the position of the inner surface of the metal at the withdrawal end by means of a capacitance.

10. The method as claimed in claim 2 wherein the position of the inner surface of the molten metal is altered by varying the gas pressure at the withdrawal end of the mould.

stasis 

1. In a method of continuously manufacturing tubing by centrifugal casting of molten metal in a mould rotating about a horizontal axis by continuously introducing molten metal into the mould, continuously cooling one end of the mould to solidify the metal at said one end into tubular form and continuously drawing off the solidified metal in tubular form from said one end; the improvement being the steps of centrifugally separating the molten metal from slag in a separating zone of the mould and subsequently passing the molten metal to the cooled end of the mould; preventing the slag from passing to the cooled end of the mould; and withdrawing the slag from the separating zone at an end thereof opposite the cooled end of the mould.
 2. The method as claimed in claim 1 further comprising the step of introducing an inert gas into the mould adjacent the cooling means to protect the clean metal from atmospheric contamination.
 3. The method as claimed in claim 1 further comprising the step of subjecting the metal in tubular form being drawn off from the mould to a drawing operation.
 4. The method as claimed in claim 1 further comprising the step of subjecting the metal in tubular form being drawn off to a rotary swaging operation.
 5. In apparatus for the centrifugal casting of tubing including a mould rotatable about a horizontal axis, the mould having a feed end into which molten metal can be continuously fed, and a withdrawal end, the withdrawal end having means for cooling the withdrawal end and the metal passing therethrough whereby the metal is solidified into tubular form, and means for continuously withdrawing the metal solidified into tubular form; the improvement comprising separating means in said mould for centrifugally separating molten metal from slag, said separating means preventing passage of slag from the feed end into the withdrawal end; and peripheral weir means at the feed end of the mould over which slag is withdrawn from the feed end as the level of slag increases.
 6. Apparatus as claimed in claim 5 wherein said separating means comprises a partition dividing the mould and having through passages radially outwardly the interface between the metal surface and slag layer.
 7. Apparatus as claimed in claim 6 wherein said partition is in the form of a refractory spider attached to the mould and a disc sealed to the spider.
 8. The method as claimed in claim 1 wherein the thickness of the metal withdrawn in tubular form is controlled by varying the position of the metal/slag interface at the feed end.
 9. The method as claimed in claim 8 comprising the further step of sensing the position of the inner surface of the metal at the withdrawal end by means of a capacitance.
 10. The method as claimed in claim 2 wherein the position of the inner surface of the molten metal is altered by varying the gas pressure at the withdrawal end of the mould. 